Overprotective of iphone?

I just bought an iphone 4 and I don't really like to let my friends use my phone. i had an i phone 3Gs before this, and I was also very protective of it. I think its because a lot of my friends are rough and their hands are not always clean. Whenever they use it I will quickly rush them and get it back. I don't want them to think I'm selfish or anything but I really don't want my phone to be dirtied. Any suggestions? :)